The present invention relates generally to the fabrication of eyeglasses, and, more particularly, to the making of eyeglass frames and parts thereof from a nickel-free titanium shape memory alloy.
Alloys used in conventional eyeglass frames include stainless steel, copper based alloys and nickel-silver.
The concept of using shape memory alloys for eyeglass components has been suggested in numerous articles and patents. Y. Suzuki, at that time head of shape memory alloy research at Furukawa Electric in Japan, published in Kinzoku Journal, vol. 31, No. 11, p115, 1981, the advantages of pseudoelastic shape memory alloy wire for fixing a lens into a frame. These findings were incorporated in one of the earliest patents on shape memory alloy applications for eyeglasses, Kokai Patent 56-89715, (Publication Date: Jul. 21, 1981) whose applications date back to 1979. Since these earlier studies, many other patents have issued claiming the advantages of using shape memory alloys for eyeglass components.
The driving force for making metal eyeglass frames from shape memory alloys lies in their great resistance to permanent deformation as compared to conventional alloys employed in this application.
Shape memory alloys belong to a class which exhibit what is termed thermoelastic martensite transformation. The term martensite refers to the crystalline phase which is produced in steels when quenched from a high temperature. The phase which exists at the elevated temperature is referred to as austenite; these terms have been carried over to describe the transformations which occur in shape memory alloys. When a steel has been quenched from the austenitic temperature to martensite, to again form austenite requires heating the structure to quite high temperatures, usually in excess of 1400xc2x0 F. By contrast, the thermoelastic shape memory alloys can change from martensite to austenite and back again on heating and cooling over a very small temperature range, typically from 18 to 55xc2x0 F. The transformation of a shape memory alloy is usually described by its hysteresis curve.
Materials which undergo martensite transformation may exhibit xe2x80x9cShape Memory Effectxe2x80x9d and xe2x80x9cPseudo-elasticity.xe2x80x9d During the transformation on cooling, the high temperature phase known as xe2x80x9caustenitexe2x80x9d changes its crystalline structure through a diffusionless shear process adopting a less symmetrical structure called xe2x80x9cmartensitexe2x80x9d, and, on heating, the reverse transformation occurs. The starting temperature of the cooling transformation is referred to as the Ms temperature and the finishing temperature, Mf. The starting and finishing temperatures of the reverse transformation on heating are referred to as As and Af respectively.
Materials exhibiting Shape Memory Effect can be deformed in their martensitic phase and upon heating recover their original shapes. These materials can also be deformed in their austenitic phase above the Af temperature through stress-induced martensitic transformation and recover their original shapes upon unloading. This strain recovery, referred to as xe2x80x9cpseudo-elasticityxe2x80x9d [sometimes referred to herein as xe2x80x9cPExe2x80x9d] is associated with the reversion of stress-induced martensite back to austenite. A well known shape memory alloy is nitinol, a near-stoichiometric alloy of nickel and titanium.
Pure titanium has an isomorphous transformation at 882xc2x0 C. The body centered cubic (bcc) structure, so called xcex2-Ti, is stable above the isomorphous point and the hexagonal close packed (hcp) structure, so called xcex1-Ti, is stable below. When alloyed with elements such as vanadium, molybdenum, or niobium, the resulting alloys have an extended xcex2 phase stability below 882xc2x0 C. On the contrary, when alloyed with elements such as Al or oxygen, the temperature range of stable xcex1 phase extends above the isomorphous point. Elements which have the effect of extending the xcex2 phase temperature range are called the xcex2 stabilizers while those capable of extending the xcex1 phase temperature range are called the xcex1 stabilizers.
For alloys with. a high enough concentration of xcex2 stabilizer elements, the material would be sufficiently stabilized to obtain a meta-stable xcex2 phase structure at room temperature. The alloys showing such a property are called xcex2 titanium alloys. Martensite transformations are commonly found among xcex2 titanium alloys. The MS temperatures in xcex2-Ti alloys decrease with increasing amount of xcex2 stabilizer in the alloys, while increasing amount of xcex1 stabilizer raises the MS. The dependence of MS on the concentration of some transition metals in binary titanium alloys is shown in FIG. 14 [xe2x80x98The Martensite Transformation Temperature in Titanium Binary Alloysxe2x80x99, Paul Duwez, Trans. ASM, vol. 45, pp.934-940, 1953]. Therefore, depending on the extent of stabilization, xcex2-Ti alloys may exhibit martensitic transformation when cooled very quickly from temperatures above the xcex2 transus, the temperatures above which xcex2 is the single phase at equilibrium.
To exhibit PE at room temperature, the alloys must be sufficiently xcex2 stabilized to have the Af point suppressed to below the ambient, but still allow the formation of stress-induced martensite before plastic deformation occurs. That is, the stress level for the martensite to form must be lower than that of plastic deformation. Shape memory effect, on the other hand, is observed when an alloy has an As point higher than and MS temperature slightly below room temperature. Stress-induced martensite transformations have also been observed in xcex2 titanium alloys [xe2x80x98Formation and Reversion of Stress Induced Martensite in Tixe2x80x9410Vxe2x80x942Fexe2x80x943Alxe2x80x99, T.W. Duerig, J Albrecht, D. Richter and P. Fischer, Acta Metall., vol. 30, pp.2161-2172, 1982].
Both shape memory effect and pseudo-elasticity have been observed in certain Tixe2x80x94Moxe2x80x94Al xcex2 titanium alloys [xe2x80x98Shape Memory Effect in Tixe2x80x94Moxe2x80x94Al Alloysxe2x80x99, Hisaoki Sasano and Toshiyuki Suzuki, Proc. 5th Int. Conf. on Titanium, Munich, Germany, pp.1667-1674, 1984]. In order to obtain SME or PE at room temperature the material has to be properly heat treated to produce the uniform xcex2 phase structure. The heat treatment to achieve that goal is called a solution treatment in which the test sample is heated to temperatures slightly above the xcex2 transus for a period of time long enough to allow for full austenization and then immediately cooled to room temperature.
Some xcex2-Ti alloys, for example, TMA (Registered trade mark of Ormco, Glendora, Calif.), has been successfully commercialized for orthodontic arch wire application. The detailed description of the applications and properties of xcex2 titanium wires can be found in U.S. Pat. No. 4,197,643. The TMA wires show a unique balance of low stiffness, high spring-back, good formability [xe2x80x98Beta titanium: A new orthodontic alloyxe2x80x99, C. Burstone and A. Jon Goldberg, American Journal of orthodontics, pp.121-132, February 1980], and weldability. [xe2x80x98Optimal welding of beta titanium orthodontic wiresxe2x80x99, Kenneth R. Nelson et al, American Journal of Orthodontics and Dentofacial Orthopedics, pp.213-219, September 1987]. The nickel-free chemistry of the alloy makes it more tolerable to some eyeglass wearers. However, TMA wires utilize the inherent mechanical properties of the material through thermo-mechanical processing. The material does not exhibit PE due to the occurrence and reversion of stress-induced martensite in the material.
Eyeglass frames fabricated from shape memory alloys are known to possess the advantages of wearer comfort and great resistance to accidental damage. The alloy traditionally used for this purpose is an equiatomic nickel-titanium alloy which exhibits pseudoelastic properties. These alloys are difficult to form, and require very exacting heat treatment to yield the properties required for eyeglass components; in addition they cannot be readily fusion welded.
An object of the present invention is to provide a titanium nickel-free SME alloy which is particularly useful for eyeglass components with no allergenic properties typical of other nickel containing PE alloys.
Another object of the present invention is to provide an alloy having pseudo-elastic properties and which is useful for eyeglass components.
A further object of the present invention is to provide super-elastic eyeglass components made from formable, weldable nickel-free shape memory alloy.
Yet a further object of the present invention is to provide nickel-free shape memory or pseudo-elastic compositions with good formabilty for the fabrication of eyeglass components.
These and other objects of the present invention are accomplished by forming eyeglasses and eyeglass components with a nickel-free xcex2 titanium alloy characterized by exhibiting pseudo-elasticity at xe2x88x9225xc2x0 C. to 50xc2x0 C. or greater due to the formation and reversion of stress-induced martensite. Such an alloy exhibits SME at room temperature when the As temperature is higher than room temperature. Furthermore, the alloy exhibits pseudo-elasticity with lower stiffness and force output magnitude, and better formability than Nitinol, the ability to be welded to other appliances, and good corrosion resistance.
It is capable of being cold worked to 20% without significantly reducing the pseudo-elastic performance, whereby it can be cold formed into various shapes at ambient temperature while retaining the high spring-back characteristics of the pseudo-elastic phenomenon, and it can be made so that it exhibits pseudo-elasticity over a wider temperature range than typical Nitinol alloys.
A nickel-free xcex2 titanium having super-elastic properties by being cold worked in its martensitic state, the alloy exhibiting complete elastic behavior at strains up to 4%, thereby permitting the designing of eyeglass components which are resistant to permanent deformation or kinking. The nickel-free xcex2 titanium alloy may be formed from:
(a) between 10.0 and 12.0 wt. % molybdenum;
(b) between 2.8 and 4.0 wt. % aluminum;
(c) chromium and vanadium between 0.0 and 2.0 wt. % chromium and vanadium; and
(d) between 0.0 and 4.0 wt. % niobium; and
(e) the balance titanium.
There may be a balanced amount of the alloying elements, and an effective amount of at least one selected from the group consisting of chromium, vanadium and niobium.
In one arrangement it may be formed of molybdenum of 10.2 wt. %, aluminum of 2.8 wt. %, vanadium of 1.8 wt. %, niobium of 3.7 wt. % and the balance of titanium and exhibit pseudo-elasticity between 25 and xe2x88x9225xc2x0 C.
In another arrangement it may be formed of molybdenum of 11.1 wt. %, aluminum of 2.95 wt. %, vanadium of 1.9 wt. %, niobium of 4.0 wt. % and the balance of titanium and exhibit pseudo-elasticity between 50 and xe2x88x9225xc2x0 C.
It is also a method for making a nickel-free xcex2 titanium alloy, comprising the steps of alloying together:
(a) between 10.0 and 12.0 wt. % molybdenum;
(b) between 2.8 and 4.0 wt. % aluminum;
(c) chromium and vanadium between 0.0 and 2.0 wt. % chromium and vanadium; and
(d) between 0.0 and 4.0 wt. % niobium; and
(e) the balance titanium.
In this method the alloy can be cold worked up to 20% without significantly reducing the pseudo-elastic performance, whereby the alloy is capable of being cold formed into various shapes at ambient temperature while retaining the high spring-back characteristics of the pseudo-elastic phenomenon.